1. Field of the Invention
The present invention relates to hydraulically powered equipment, and more particularly to apparatus for reducing bounce of a hydraulically driven member that is stopped suddenly.
2. Description of the Related Art
With reference to FIG. 1, a backhoe 10 is a common type of earth moving equipment that has a bucket 12 attached to the end of an arm 14 which in turn is coupled by a boom 15 to a tractor 18. A pivot joint 16 enables backhoe assembly 17 formed by the combination of the bucket, arm, and boom to pivot left and right with respect to the rear end of the tractor 18. A pair of hydraulic cylinders 19 are attached to the boom 15 on opposite sides of the backhoe tractor 18 and provide the drive force for the pivotal action. Hydraulic fluid is supplied to the cylinders 19 through control valves that are manipulated by the backhoe operator. The pivotal movement of the boom 15 is referred to as “swing” or “slew”.
As the boom 15 slews, pressurized fluid is introduced into one chamber of each cylinder, referred to as the “driving chamber”, and fluid is drained from the other cylinder chamber, referred to as the “exhausting chamber”. Due to the mass of the boom and any load being carried, a significant amount of kinetic energy is associated with its motion. When an operator terminates slewing at a rapid pace by releasing the handle attached to the control valve, the energy associated with the boom's motion has to dissipate in order for the system to return to an “at-rest” state (the state of minimal energy). With a conventional control valve assembly, pressure in the former exhausting chambers of the swing cylinders 19 increases as the boom 15 continues to move in the driven direction, due to inertia. As this pressure continues increasing, a pressure relief valve typically is activated to prevent the cylinder pressures from reaching a dangerous level. This caused pressure in the driving cylinder chambers to decrease.
At this time point, there is a net pressure difference between the two chambers of each cylinder 19 which causes the direction of motion to reverse. As the motion reverses, the pressure relief valve closes trapping pressure in the former exhausting chambers and associated hydraulic lines. The trapped pressure begins to decay as the boom 15 now is being driven in the opposite direction which expands the former exhausting chambers and causes a rise in pressure in the former driving chambers of the cylinders. Eventually the pressure the former driving chambers becomes significantly greater than pressure in the former exhausting chambers resulting in another reversal of boom motion. The boom 15 oscillates, initially activating the pressure relief valves, but later just cycling back and forth, until the energy is dissipated to the environment through heat, sound, material hysteresis, etc. This phenomenon is known as “slew bounce” or “slew wag” and increases the time required to properly position the boom 15. As a consequence, it adversely affects equipment productivity.
Various approaches have been devised to minimize the slew bounce. For example, U.S. Pat. No. 4,757,685 employs a separate relief valve for each hydraulic line connected to the swing cylinders, which valves vent fluid to a tank return conduit when excessive pressure occurs in those cylinders. Additional fluid is supplied from the tank return conduit through a make-up valve when a cylinder chamber cavitates. This system also incorporates a means for communicating pressurized fluid from the pump supply line to the tank return conduit when an operator slew control valve is in the neutral position.
U.S. Pat. No. 5,025,626 describes a cushioned swing circuit which also has relief and make-up valves connected to the hydraulic lines for the slew cylinders. This circuit also incorporates a cushion valve which in an open position provides a fluid path between the cylinder hydraulic lines. That path includes a flow restriction orifice. The cushion valve is biased into the closed position by a spring and a mechanism opens the cushion valve for a predetermined time period when the pressure differential between the cylinder chambers exceeds a given threshold.